What is the difference between Hong Kong and China?

There is a big difference between Hong Kong and Mainland China.

Hong Kong is a former British colony. The official language is Chinese and English. Like Britain, to make an emergency call to the police, ambulance or fire service is to dial 999.
Although Hong Kong is part of China now but it is called a Special Administrative District. Things will be remained the same for 50 years counting from 1997 .( the year of the handover ) So basically everything is the same as it was when as a British colony. People are free to speak , free to do whatever they want providing that it's legal. People have access to western culture and fashion as before. Food has many variety. Mainly Chinese population, they usually will have Chinese meal but you will find all kinds of restaurants serving different cuisines, such as Japanese, Thai, Greek, French, Vietnam, Spanish, Portuguese , you name it.
People enjoy life as a democratic city. They work hard and they have a lot of entertainments such as shops opened till 10- midnight, tubes are run till 1:ooam or all night during Xmas and new year. Every area has 7-11 24 hours shop. Transport is convenient that's why very few people bother to own a car. Why give yourself headache when there is 24 hours mini 14 seats van service everywhere ? Even taxi is a very cheap transport. As for needing a car to carry your shopping, Hong Kong people don't do like the westerner, going to supermarket once a week and shop for the whole week. They shop for fresh food in a real market everyday. When you shop for fresh vegetable , meat or seafood everyday you don't need a car, all you carry is only a few bags.
Hong Kong doesn't particularly produces anything. It's a financial metropolitan city. It's Hang Seng index in the stock exchange can affect the whole world.It's also a materialistic society, work hard and enjoy life, buy whatever designer's good you can afford, go abroad for holiday when you can get times off from work, live to a full life --that's the motto of most people.

Now about Mainland China,that's a totally different kettle of fish. People there all dying to go and stay in Hong Kong but it's not that easy. The fact that Hong Kong and China are the same country now doesn't entitle people from Mainland China just pack their bag and move to Hong Kong. They need to apply a document called single journey license or return journey license, the latter one means they have to return to China at a certain time, usually after 3 months. Unless they marry a Hong Kong citizen, in which case they still have to travel back and fro every 3 months between HK and MC for 7 years until they get the Hong Kong Identity card.
People in Mainland China have a different mentality. They want to leave China and go for a real free and democratic country.As there is still corruption and restriction of speech and political views. The nearest is Hong Kong but if they can they will go to the west. Food in China are mainly Chinese cuisine but still if you can afford there are restaurants serving all kinds of cuisines in big cities such as Shanghai and Peking. In rural area there are still people suffered from proverty, illiteracy, under nutrited. All people can see about the blooming of China are those you see in cities only. China is a big country. There are many deserted mountain area or forlorn villages even Chinese never heard about them. Some still live without electricity or proper water supply.
As for transport, like everywhere, there are tubes, taxi,buses in all cities but in some countryside people will make do with bicycle or even walking.
Mandarin is the official language but different towns and cities have their own different dialects. The people from Guangchow ( in the south, near Hong Kong) and Shenzhen ( border city between HK and China) speak Cantonese like Hong Kong people do.